Contactless current measurement can be accomplished by using a magnetic field sensor to sense the magnetic field caused by a current passing through a primary conductor (further referred to as primary current). The magnetic field caused by the primary current depends on the magnitude of the primary current. For the example of a long straight wire carrying a primary current iP the magnitude of the resulting magnetic field H at a distance d from the wire is directly proportional to the primary current iP. In accordance to the Biot-Savart law, the magnitude of the magnetic field H equals H=iP/(2πd) if the wire is very long (theoretically infinitely long) as compared to the distance d. In practice, a chip package, which includes the magnetic field sensor element (e.g. a Hall sensor) is placed closely to the primary conductor. This chip package is also referred to as sensor package or current sensor package. The magnetic field sensor element (or short: magnetic field sensor) included in the chip package is thus exposed to the magnetic field caused by the primary current, and the sensor signal (usually a voltage signal) provided by the magnetic field sensor element is proportional to the magnitude of the magnetic field and thus also proportional to the primary current.
Usually Hall sensors and magneto-resistive sensors are used for contactless current measurement. Magneto-resistive sensors are often referred to as XMR sensors, which is a collective term for anisotropic magneto-resistive (AMR), giant magneto-resistive (GMR), tunneling magneto-resistive (TMR), and colossal magneto-resistive (CMR) sensors. The sensor package may also include a signal processing circuit that receives the signal from the magnetic field sensor element (referred to as sensor signal) and derives, from the sensor signal, a measurement signal that represents the primary current.
A measuring device for contactless current measurement (also referred to as current sensor) usually includes a soft-magnetic core or a magnetic flux concentrator in order to direct the magnetic field, which is caused by the primary current, onto the magnetic field sensor element. The soft-magnetic core may also shield the magnetic field sensor element from disturbing external magnetic fields, which may cause measuring errors. However, using a soft-magnetic core of flux concentrators may give rise to undesired effects due to the non-linear characteristic and the hysteresis of the soft-magnetic core. The hysteresis of the core may lead to a zero-point error in the current measurement. To avoid such problems, coreless current sensors have been developed. However, when using coreless current sensors a precise positioning of the magnetic field sensor element relative to the primary conductor is crucial for an accurate current measurement. Thus, tolerances for the assembly of the current sensor have to be relatively tight.
Generally, the accuracy of coreless magnetic current measurement is affected by the geometry of the primary conductor (also referred to as current rail) and by the position of primary conductor relative to the magnetic field sensor element. To ensure a precise relative position of primary conductor and magnetic field sensor element, the primary conductor and the magnetic field sensor element can be integrated in the same chip package (sensor package). Such an approach is used, for example, in the TLI4970 current sensor family of Infineon. When using a current sensor with an integrated current rail, the primary conductor (e.g. a wire, a cable, a current bar, etc.) carrying the primary current has to be disconnected (interrupted) in order to insert the chip package with the integrated current rail. In many applications, disconnecting the primary conductor is either undesired or impossible. When using a primary conductor, which is fully external to the chip package including the magnetic field sensor element, the problem remains that the measured signal (representing the primary current) will heavily depend on the geometry of the assembly of primary conductor and chip package without precisely knowing the transfer characteristic (primary current to measured signal) of the measurement set-up. Thus there is a general need for improved current sensors which allow a precise coreless and contactless current measurement.